Nitride semiconductors have properties, such as high saturation electron velocity and wide band gaps. Thus, the use of nitride semiconductors for high-power semiconductor devices with high breakdown voltages has been studied. For example, GaN, which is a nitride semiconductor, has a band gap of 3.4 eV, which is larger than the band gap of Si (1.1 eV) and the band gap of GaAs (1.4 eV). Hence, GaN has a high breakdown field strength. Accordingly, GaN is a highly promising material for high-power semiconductor devices, which operate at high voltages, used in power supplies.
There have been many reports about field-effect transistors, in particular, high electron mobility transistors (HEMTs) serving as semiconductor devices made of nitride semiconductors. For example, among GaN-based HEMTs (GaN-HEMTs), AlGaN/GaN HEMTs including an electron transit layer made of GaN and an electron supply layer made of AlGaN have been receiving attention. In an AlGaN/GaN HEMT, a difference in lattice constant between GaN and AlGaN causes strain in AlGaN. The strain-induced piezoelectric polarization and the spontaneous polarization of AlGaN result in a high-density two-dimensional electron gas (2 DEG). Thus, AlGaN/GaN HEMTs are promising high-breakdown voltage power devices for, for example, high-efficiency switch elements and electric vehicles.
Although nitride semiconductor devices are promising high-breakdown voltage power devices, a gate leakage current increases due to crystal defects of a nitride semiconductor during the high-power operation, which decreases the efficiency and results in unstable performance. To address this problem, there has been proposed a method for suppressing a gate leakage current by forming a GaN cap layer on an AlGaN electron supply layer. However, this method poses a problem in that growth is interrupted because of different optimum growth conditions between AlGaN and GaN, and thus a defect is formed at an interface between the electron supply layer and the cap layer. Therefore, there has been proposed another method for suppressing a gate leakage current by oxidizing a surface of the cap layer through an oxygen plasma treatment. However, this method poses a problem in that a defect is formed in the cap layer because of the oxidation of the surface of the cap layer, which causes, for example, current collapse and degrades the performance of the device.
The followings are reference documents.    [Document 1] P. Ivo et al., “Influence of GaN cap on robustness of AlGaN/GaN HEMTs”, in Proc. IEEE Int. Reliab. Phys. Symp., 2009, pp. 71-75, and    [Document 2] J. W. Chung et al., “Effect of gate leakage in the subthreshold characteristics of AlGaN/GaN HEMTs”, IEEE Electron Device Lett., vol. 29, no. 11, pp. 1196-1198, November 2008.